238 



may vary considerably, but in all cases the ferro-magnesian minerals are 

 present in much greater abundance than in the normal proterobases and 

 epidiorites. The opacite or granular leucoxene often lies in bands which 

 curve round the spots of interlacing actinolite. Under ordinary light the 

 chlorite and hornblende cannot be easily distinguished ; but under crossed 

 nicols the distinction becomes easy as the former mineral gives only a dark 

 blue shimmering light, whereas the latter polarizes in much higher tints. 

 An important question arises as to the significance of the spots. Do 

 they represent original vesicles ? 



Mr. PHILLIPS states that DE LA BECHE regarded these rocks as highly 

 altered ash beds and remarks that it is not improbable that some of them 

 may have been originally flows of volcanic inud. The rocks which run 

 out to sea west of St. Michael's Mount (marked the Hogus on the map) 

 appear to belong to this group, at any rate in part, and they are interest- 

 ing as giving decided evidence of fragmental origin. Highly basic 

 hornblende-slates of the above type occur at Paul Hill, Eose Hill near 

 Castle Horneck, Chyandower near Penzance, and Rosemorran near Gulval. 

 The following analyses by Mr. PHILLIPS illustrate the composition of the 

 different types of " greenstone " above referred to. 



PROTEROBASES AND EPIDIORITES. 

 I. II. III. IV. V. VI. 



99-88 99-66 100-04 100-01 99'90 100-34 



Sp. Gr. 3.02 3.01 2.98 2.97 3.04 2.90 



I." Rock immediately north of St. Peter's Vicarage, Tolcarn. [Proterobase.] 

 II. Fine-grained crystalline rock from Tolcarn quarry. 



III. Chapel Rock, west of Causeway leading to St. Michael's Mount. [Epidiorite.] 



IV. Rock from the Sanctuary quarry near St. Austell. Also contains ! 3 of lithia. [Pro- 

 terobase.] 



V. Trewint, near Five Lanes, north-east Cornwall. [Epidiorite.] 



VI. Coarse grained crystalline rock from St. Cleer Down, south-east Cornwall] 

 Proterobase,] 



